Discrete dynode detector with dynamic gain control

ABSTRACT

A novel electron multiplier that regulates in real time the gain of downstream dynodes as the instrument receives input signals is introduced. In particular, the methods, electron multiplier structures, and coupled control circuits of the present invention enable a resultant on the fly control signal to be generated upon receiving a predetermined threshold detection signal so as to enable the voltage regulation of one or more downstream dynodes near the output of the device. Accordingly, such a novel design, as presented herein, prevents the dynodes near the output of the instrument from being exposed to deleterious current pulses that can accelerate the aging process of the dynode structures that are essential to the device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of particle detectorinstrumentation. More particularly, the present invention relates toelectron multipliers wherein real time signal generated by anintermediate dynode is monitored to regulate in real time the gain todynodes near the output of the instrument.

2. Discussion of the Related Art

Electron multipliers are often utilized as detectors for the detectionof particles such as photons, neutral molecules, and/or ions provided bymass spectrometry. While the geometry of such devices can vary, a commonbeneficial design comprises a cathode, an anode, and a chain ofresistors and capacitors coupled to a plurality of about 10-25 discreteelectron multiplier disposed structures (dynodes). Collectively, such anarrangement provides a plurality of stages that when operated withvoltages between about 1000-5000V enable gains often between about than10⁵ up to about 10⁷. Beneficially, the dynode (discrete) geometry andoperating parameters is often utilized as part of an ion detector whenconfigured to operate with mass analyzers, such as, but not limited tomass filters, ion traps, and time of flight mass spectrometers wheresignificant variation of ion flux is common when operating in variousscanning modes or when recording time transients.

The potential difference between a pair of dynodes is often designed sothat an electron striking a dynode can produce more than one secondaryelectron. The average number of secondary electrons per primary electronproduced at a particular dynode is the gain of that stage of theelectron multiplier with the gain of the entire electron multiplierbeing the product of the gain at every stage from the cathode to thelast dynode. Increasing the voltage applied to the electron multipliertypically increases the voltage between dynodes, increasing the gain ofeach stage, thereby increasing the gain of the entire multiplier.Operating these detectors with high gain is desired for the detection oflow-level signals in order to improve signal-to-noise ratio. However,such high gain values and thus high secondary fluxes result in intenseelectron currents in the final stages of the current amplification alongthe various dynode structures. While the use of discrete dynodearchitecture allows for better control of individual dynodes in thefinal part of the chain, the beneficial design still cannot in totalprevent strong electron currents from hitting dynodes near the output ofthe device.

One of the primary reasons for aging when utilizing discrete dynodeelectron multipliers is the carbon deposition on the surface of one ormore dynodes which are adjacent the anode of the instrument. Theaccumulation of excessive carbon deposition has been attributed to thehigher doses per unit area of secondary electrons from the dynodes nearthe anode that enable a carbon to become bonded to the dynode surfaces,which reduces the secondary yield. As part of the phenomenon, thedeleterious buildup of carbon occurs more rapidly in poor vacuumconditions, most typical of ion trap instruments. Those of ordinaryskill in the art have applied approaches to resolve this issue toinclude: 1) lowering the background pressure to reduce the carbonbuildup; 2) increasing the active surface area of the dynodes underelectron impact; and 3) disassembly of the dynodes structures to cleanand/or refurbish the device. However, while such approaches have beenshown to somewhat ameliorate the aging process of the dynode structures,they are often not always desirable because of the technical challengesand associated costs.

Background information for an electron multiplier that limits theresponse of the instrument when subjected to a large input signal for aninitial period of time, is described and claimed in, U.S. Pat. No.6,841,936, entitled, “FAST RECOVERY ELECTRON MULTIPLIER,” issued Jan.11, 2005, to Keller et al., including the following, “[a]n improvedelectron multiplier bias network that limits the response of themultiplier when the multiplier is faced with very large input signals,but then permits the multiplier to recover quickly following the largeinput signal. In one aspect, this invention provides an electronmultiplier, having a cathode that emits electrons in response toreceiving a particle, wherein the particle is one of a charged particle,a neutral particle, or a photon; an ordered chain of dynodes whereineach dynode receives electrons from a preceding dynode and emits alarger number of electrons to be received by the next dynode in thechain, wherein the first dynode of the ordered chain of dynodes receiveselectrons emitted by the cathode; an anode that collects the electronsemitted by the last dynode of the ordered chain of dynodes; a biasingsystem that biases each dynode of the ordered chain of dynodes to aspecific potential; a set of charge reservoirs, wherein each chargereservoir of the set of charge reservoirs is connected with one of thedynodes of the ordered chain of dynodes; and an isolating element placedbetween one of the dynodes and its corresponding charge reservoir, wherethe isolating element is configured to control the response of theelectron multiplier when the multiplier receives a large input signal,so as to permit the multiplier to enter into and exit from saturation ina controlled and rapid manner.”

Background information for a photomultiplier detector that includes again control circuit to provide feedback to a dynode situated near theanode, is described and claimed in, U.S. Pat. No. 5,367,222, entitled,“REMOTE GAIN CONTROL CIRCUIT FOR PHOTOMULTIPLIER TUBES,” issued Nov. 22,1994, to David M. Binkley, including the following, “[a] gain controlcircuit (10) for remotely controlling the gain of a photomultiplier tube(PMT (12)). The remote gain control circuit (10) may be used with a PMT(12) having any selected number of dynodes (DY). The remote gain controlcircuit (10) is connected to the last dynode nearest the anode (16) inthe dynode string which controls the total dynode supply voltage andinfluences the gain of each dynode (DY). The remote gain control circuit(10) of the present invention includes an integrated-circuit operationalamplifier (U1), a high-voltage transistor (Q1), a plurality of resistors(R), a plurality of capacitors (C), and a plurality of diodes (D).Negative feedback is used to set the last dynode voltage proportional toa voltage controlled by the gain control voltage delivered by a voltagesource such as a digital-to-analog converter. The control circuit (10)of the present invention is connected to the last dynode using a singleconnecting wire (22).”

Background information for a photomultiplier detector having gaincontrol through change of the bias on at least one of the dynodes, isdescribed and claimed in, U.S. Pat. No. 4,804,891, entitled,“PHOTOMULTIPLIER TUBE WITH GAIN CONTROL,” issued Feb. 14, 1989, toHarold E. Sweeney, including the following, “[i]mproved gain control ina photomultiplier tube having a plurality of dynode stages is achievedthrough manual or automatic change of the bias voltage on at least oneof the several dynodes between the anode and cathode of the tube. Bysuch means, maximum tube gain change is obtained with a minimum of biasvoltage swing.”

Background information for a photomultiplier detector having automaticgain control, is described and claimed in, U.S. Pat. No. 3,614,646,entitled, “PHOTOMULTIPLIER TUBE AGC USING PHOTOEMITTER-SENSOR FRO DYNODEBIASING,” issued Oct. 19, 1971, to Earl T. Hansen, including thefollowing, “[a] photomultiplier tube automatic gain control circuitwherein the biasing potentials between a plurality of adjacent dynodesare varied inversely as the amplitude of the photomultiplier outputsignal. The output signal is detected and applied to aphotoemitter-sensor connected in shunt with the biasing network for theaforesaid dynodes.”

Accordingly, there is a need in the field of particle detection toimprove the operational lifetime for such structures when operated athigh gains. The present invention addresses this need, as disclosedherein, by providing a novel intermediate dynode structure and coupledcircuit to regulate the gain and thus the intensity of the secondaryemission of one or more downstream dynodes near the output of the deviceno matter high strong of an input signal.

SUMMARY OF THE INVENTION

The present invention is directed to a novel particle detector thatincludes a cathode that emits electrons in response to receivingincident particles that represent one or more input signals; a pluralityof cascaded dynodes configured to provide a sensed current at an anodethat is related to the number of received incident particles; aninterposed partitioned dynode arranged as part of the plurality ofcascaded dynodes to provide a detection current indicative of themagnitude of the one or more input signals; and a control circuitcoupled to one or more downstream dynodes within the plurality ofcascade dynodes and configured to receive the detection current so as toregulate the voltage gain to the one or more downstream dynodes upon thedetection current being above a predetermined threshold.

Accordingly, the present invention provides for an apparatus and methodof operation that enables a user to prevent high current pulses (beyondthe capacity of a dynode capacitor combination) to hit the dynodes. Inparticular, the methods, electron multiplier structures, and coupledcontrol circuits of the present invention enable a resultant on the flycontrol signal to be generated upon receiving a predetermined thresholddetection signal so as to enable the voltage regulation of one or moredownstream dynodes near the output of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a beneficial example detector of the present invention.

FIG. 1B shows the dynode arrangement with an example coupled resistivenetwork.

FIG. 2 shows a plot of the available current and slew rate operatingparameters from Table 1 when using a 25 dynode electron multiplierdesign and a 90%-10% partition ratio for the modified intermediateelectrode.

DETAILED DESCRIPTION

In the description of the invention herein, it is understood that a wordappearing in the singular encompasses its plural counterpart, and a wordappearing in the plural encompasses its singular counterpart, unlessimplicitly or explicitly understood or stated otherwise. Furthermore, itis understood that for any given component or embodiment describedherein, any of the possible candidates or alternatives listed for thatcomponent may generally be used individually or in combination with oneanother, unless implicitly or explicitly understood or stated otherwise.Moreover, it is to be appreciated that the figures, as shown herein, arenot necessarily drawn to scale, wherein some of the elements may bedrawn merely for clarity of the invention. Also, reference numerals maybe repeated among the various figures to show corresponding or analogouselements. Additionally, it will be understood that any list of suchcandidates or alternatives is merely illustrative, not limiting, unlessimplicitly or explicitly understood or stated otherwise. In addition,unless otherwise indicated, numbers expressing quantities ofingredients, constituents, reaction conditions and so forth used in thespecification and claims are to be understood as being modified by theterm “about.”

Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and attached claims are approximationsthat may vary depending upon the desired properties sought to beobtained by the subject matter presented herein. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the subject matter presented herein are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical values, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

General Description

The present invention is directed to a detector design wherein desireddynode structures, often the final dynodes of an electron multiplierdetector, are prevented from being subjected to high current pulses evenin the event of high input signals. To enable such a result, a detectionsignal, as disclosed herein, is provided by a modified intermediatedynode that enables a coupled regulating circuit to adjust the gain tothe one or more downstream dynodes.

The surface area of the novel intermediate dynode within the cascadedladder of dynodes has its surface emitting area partitioned such thatelectron current impacts the partitioned areas in the ratio of about50%-50% up to about a ratio of 95%-5%, more often in a ratio of about90%-10%. Electrons hitting the equal or larger area are allowed topropagate in a normal mode, i.e., from dynode to dynode, while thosehitting the remaining equal or smaller area provides the current to beevaluated by a coupled regulating control circuit.

The coupled regulating circuit can simultaneously in real time evaluatethe provided for current (i.e., the detection signal) from theintermediate dynode and if the measured current exceeds a predeterminedthreshold, it can generate a desired corresponding control signal. Inparticular, the resultant detection signal provided from an intermediatedynode is utilized in a time constrained fashion to enable a regulatingcontrol circuit to switch the gain voltage on one or more dynodesadjacent to the anode. The important criteria is that such a detectionsignal (a current signal) if above a predetermined threshold value andthe corresponding switching aspect (i.e., the control signal) isproperly administered to the desired dynode(s) before the arrival of thenormally propagating electron current that is moving along the longerelectron pathway (i.e., from dynode to dynode). Accordingly, theconfiguration and method of the present invention in a novel fashion candynamically drop the gain at the desired dynode(s) so as to preventunnecessary current amplification that if left unchecked can contributeto, for example, undesirable contamination effects.

Specific Description

The Detector

FIG. 1A shows a basic non-limiting beneficial example embodiment of adiscrete dynode detector, generally designated by the reference numeral10 that can be used with the methods of the present invention. It is tobe understood that the detector described herein is capable of detectingparticles selected from photons, neutral molecules, as well as ionsprovided by any mass spectrometer instrument that can provide an ionpath to be received by the detector of the present invention. Exampledetectors include photomultipliers and discreet dynode instruments.Example mass spectrometer instruments are to include, but are notlimited to time of flight instruments (TOFs), and quadrupole electrodedevices (e.g., ion traps).

In general, FIG. 1A shows a detector 10 that comprises an electronmultiplier section 1 (as shown in the solid rectangular box) thatfurther includes a plurality of cascaded dynodes 3, a novel intermediatedetection dynode 5 to provide a detection signal (current), and acoupled control circuit 12 (shown enclosed in a dashed box) configuredto regulate one or more downstream dynodes 6 (e.g., D_(—n-1) andD_(—n-2)), which can include the last dynode 7 (D_(—n)) in the chain,that make up the plurality of cascaded dynodes 3.

As is known by those skilled in the art, discrete dynode multipliers,such as exemplified by the detector of FIG. 1A, operate on the principleof secondary electron emission. The greater the number of dynodes, thelonger the overall normal propagating pathway as well as a greater gainbecause each dynode increases the number of multiplied electrons movingalong a path from dynode to dynode. As shown in the example embodimentof FIG. 1A, the plurality of dynodes, generally designated by thereference numeral 1 includes two rows of dynodes, which rows extendgenerally but not necessarily parallel to each other from an input end(i.e., as generally provided at dynode D1 4) to an output collectoranode end 8 to provide a desired sensed signal indicative of one or moreinputs signals. It is to be understood that the number of dynodes(electrodes) of the present invention can vary from about 13 up to about40 dynodes in the detector 10 arrangement as long as the overall gain isadequate to provide the requisite signal to noise ratio and as long asthe chosen intermediate dynode(s) can provide a necessary sensingcurrent with a spatial separation from the desired downstream dynode(s)to allow time to dynamically regulate the voltages of the detector 10when appropriate.

To illustrate operability of the detector 10 shown in FIG. 1A, thedynode electron multiplier 1, as part of the detector 10, often has acoupled negative high voltage to the cathode (not shown) with respect tothe collector 8 (anode), which as one arrangement can be biased atground. The plurality of dynodes collectively shown by the referencenumeral 3 and also denoted as D₁, D₂, D₃, D_(I), etc., are thusconfigured as a series of dynodes with coupled progressively increasingpositive voltages as provided by, for example, a biasing resistivenetwork 9, as shown in FIG. 1B in addition to other discrete devices(not shown) known in the art, e.g., capacitors, zener diodes, etc., thatenable the predetermined voltages to be applied between pairs of suchdynodes 3 in a controlled fashion to thereby enable an overall gain.

FIG. 1B thus shows an example arrangement of a plurality of resistiveelements 9, that in operation comprise a voltage divider circuit for theplurality of dynodes (denoted as Dynode 1, 2, n, etc.), as also shown inthe corresponding dynode arrangement 1 of FIG. 1A. As anotherarrangement, the resistive elements 9 may be replaced with a variableresistor to also adjust the voltages between desired pairs of dynodes 3.Moreover, such resistive elements 9 can be configured as a monolithicthick-film resistor chain specifically designed for a desiredapplication to achieve the best dynamic range and lifetime for thedetector 10. In addition, the aforementioned capacitors (not shown) canbe coupled in parallel to the resistive network (resistive elements 9)in a known manner so as to prevent, as one application, unwanted voltagechanges to any of the dynode pairs (e.g., between D₁ and D₂) duringoperation. Moreover, utilized zener dynodes can be implemented to clampdesired voltages to any of the dynode pairs, more often dynodes near theanode 8 so as to protect the detector 10 from voltage spikes.

Turning back exclusively to FIG. 1A, a user in operation of the detector10 can first enable one or more particles 2 indicative of an inputsignal to be received by the first dynode 4 (also denoted as D₁) of theplurality of dynodes 3. The impact of the particle(s) 2 with the firstdynode (D1) 4 thus causes the emission of secondary electrons with thegain determined by the gain coefficient and the voltage of the cathode(not shown) that precedes it. As the secondary electrons reachsubsequent dynodes, the amount of secondary emission (i.e., the gain)and thus the corresponding current increases from dynode to dynode withan overall sensed current (I) at the anode 8 determined by Equation 1:I=qNG;  (1)wherein q is the charge on an electron, N is the number of particles(e.g., ions) per second being detected, and G is the gain of themultiplier.

Thus the current, using ions provided by a mass spectrometer as anexample, is directly related to the number of received ions detected aswell as the overall operating gain of the detector 10. However, thedynodes at the end of the chain 6, e.g., D_(—n-1) and D_(—n-2), innormal operation are impacted with the higher levels of current based onthe architecture of the dynode assembly 1. To prevent such dynodes nearthe output anode 8 from being hit with high current pulses as enabled byan input signal, the present invention provides for an intermediatedynode 5 (also denoted as D_(I)) modified to provide a current detectionsignal so as to be utilized to adjust the gain of one or more downstreamdynodes 6, e.g., D_(—n-1) and D_(—n-2). Specifically, by utilizing anintermediate dynode 5 to provide a sampled current related to the inputsignal, such a detection signal can be utilized to regulate in real-timethe gain to those downstream dynodes that can be impacted with highamounts of current if the sampled detection current is above apredetermined threshold limit.

It is to be first appreciated that the majority of configured dynodes 3in addition to the modified intermediate dynode 5 of the presentinvention can be configured as a system of rings, venetian blind-likestructures, plates, curved or planar structures that are ofteninterlaced electrodes so as to receive and direct a desired electronbundle. Moreover, the electrodes (i.e., dynodes) themselves can beconfigured with surface areas that comprise spherical structures,cylindrical structures, meshes, planar or curved strips of metalstructures, polished structures, and/or removable emissive surfacescoupled to a base material. In addition, the dynode emissive surfaces ofthe dynodes may be enhanced as understood by those of skill in the artby surface treatment from beryllium-copper or silver-magnesium materialor beneficial aluminum containing materials, such as aluminum oxide(Al₂O₃), which has been shown to be air stable and substantiallyresistant to corrosive atmospheres to result in very robust electrodes.

In whatever beneficial shape that is chosen for the intermediate dynode5 of the present invention, such a novel intermediate detection dynodeis beneficially partitioned (e.g., splitting the receiving area of theintermediate dynode into sections) so as to result in an often unequalpartitioned surface emission area in a ratio of about 95%-5%, more oftenin a ratio of about 90%-10%. Electrons hitting, for example, the largerarea are thus allowed to propagate in a normal mode, i.e., from dynodeto dynode, while those hitting the remaining smaller area provides for asampling detection signal current to be evaluated by the coupledregulating control circuit 12, as shown in FIG. 1A. It is also to beappreciated that while a single partitioned electrode is often desired,other beneficial configurations, such as interstitial designs, i.e.,electrodes having an empty space or gap between conductive areas thatinclude mesh electrodes, can also be integrated as part of theintermediate dynode when configured with other aspects of the presentinvention. For example, the mesh grid itself can configured at a firstpotential to receive and direct electrons to propagate in a normalfashion while an adjacently coupled electrode and at a differentpotential from the mesh electrode receives those electrons that aredirected through the mesh to provide for the detection current asdescribed herein. In any configuration, the intermediate dynode 5, asshown in FIG. 1A, is interposed within the chain of dynodes 3 to detecta prescribed partitioned current that is indicative of the one or moreinput signals. The detection signal in such a novel configuration canthus be beneficially received (denoted by the letter A and accompanyingdirectional arrow) by a control circuit 12 (as shown within the dashedbox of FIG. 1A) using any configuration of discreet device architecturethat is well known in the electronic arts.

As a non-limiting general example of the control circuitry 12illustrated in FIG. 1A, the detection current signal A can be firstreceived and converted by any simple current-to-voltage converter, suchas, an example trans-impedance amplifier 14 shown in FIG. 1A.Thereafter, the converted voltage signal can be directed (as denotedalong line B) into a unidirectional voltage control circuit (e.g., anerror amplifier 18) wherein the output voltage is compared to a stablereference threshold voltage. Any difference between the two generates acompensating error voltage which tends to move the output voltage(denoted along lines C shown with accompanying directional arrows) fromamplifiers 20 towards the design specification as to regulate the gainof one or more downstream dynodes 6, e.g., D_(—n-1) and D_(—n-2) so asto if required, minimize unnecessary intense electron currents. Whilethe regulating circuit 12 shown in FIG. 1A and as described herein isshown coupled to the intermediate positioned dynode 5 (denoted asD_(I)), it is to be understood that such a regulating circuit 12 of thepresent invention can be coupled to any selected dynode 3 that iscapable of being modified and arranged in the chain of dynodes whenmeeting the specifications described herein.

It is to also be appreciated that the choice of location of thepredetermined intermediate modified dynode is a compromise between thesensitivity and available slew rate of the control circuit 12 of FIG. 1Aas discussed below. Detecting upward in the dynode ladder provides alower current through the 5% up to about 50%, more often the 10%electrically coupled pick-off portion of the chosen electrode area withthe tradeoff being that a detection signal provided earlier enables moretime to switch the voltage on the regulated dynode structure that is asclose as possible to the output.

In particular, it would be beneficial to regulate the voltage on thedynode that is as close as possible to the output (e.g., a dynode 6, 7adjacent the anode 8 of FIG. 1A). Such an arrangement can enable highsensitivity of the current sensing partition (area for current sensingcan be reduced) with the limitation being the travel time of electronsand the speed of the control circuit 12.

Table 1 shown below is an illustrative resultant spreadsheet of anon-limiting example circuit model configuration, similar to that shownin FIG. 1A, listing possible operating parameters and thus designconsiderations so as to illustrate the novelty of the present invention.The listed operating parameters for this example are obtained from anexample electron multiplier detector arranged with 25 dynodes, a totalgain of about 10⁶, a coupled example capacitance measured at the 25^(th)dynode being at about 370 pF, and an acceleration field between any twodynodes being about 70 volts. Using such constraints, column 1 of Table1 shows the dynode # (i.e., the possible detection dynode), column 2shows the available current from the respective dynode when configuredwith a 10% current partition, column 3 shows the distance from dynode todynode as well as the overall path-length for the electron travel to thelast dynode, column 4 shows the response time to change the gain of the25^(th) dynode if a particular dynode is utilized to provide thedetection signal, column 5 shows the current out, and column 6 shows theslew rate (kV/μs) required to switch the 25^(th) dynode.

TABLE 1 Distance of Detection Current Detection Available Dynode To LastResponse Out Slew Rate Dynode Current Dynode (mm) Time (sec) (Amps)(kV/μs) 25 1.00E−04 0 0 — INF 24 5.76E−05 10 2.01613E−09 5.5056 14.88 233.31E−05 20 4.03226E−09 2.7528 7.44 22 1.91E−05 30 6.04839E−09 1.83524.96 21 1.10E−05 40 8.06452E−09 1.3764 3.72 20 6.32E−06 50 1.00806E−081.10112 2.976 19 3.64E−06 60 1.20968E−08 0.9176 2.48 18 2.10E−06 701.41129E−08 0.786514286 2.125714286 17 1.21E−06 80  1.6129E−08 0.68821.86 16 6.95E−07 90 1.81452E−08 0.611733333 1.653333333 15 4.00E−07 1002.01613E−08 0.55056 1.488 14 2.30E−07 110 2.21774E−08 0.5005090911.352727273 13 1.33E−07 120 2.41935E−08 0.4588 1.24 12 7.63E−08 1302.62097E−08 0.423507692 1.144615385 11 4.39E−08 140 2.82258E−080.393257143 1.062857143 10 2.53E−08 150 3.02419E−08 0.36704 0.992 91.46E−08 160 3.22581E−08 0.3441 0.93 8 8.38E−09 170 3.42742E−080.323858824 0.875294118 7 4.83E−09 180 3.62903E−08 0.3058666670.826666667 6 2.78E−09 190 3.83065E−08 0.289768421 0.783157895 51.60E−09 200 4.03226E−08 0.27528 0.744 4 9.21E−10 210 4.23387E−080.262171429 0.708571429 3 5.30E−10 220 4.43548E−08 0.2502545450.676363636 2 3.05E−10 230  4.6371E−08 0.239373913 0.646956522

To provide an understanding in the formulation of the operatingparameters that make up Table 1, Dynode 13 is chosen for illustrativepurposes as the detection dynode and thus the operating parameters forthe row comprising Dynode 13 is shown bolded for convenience so as toaid in the following discussion.

It is to be appreciated that for this non-limiting example, thedetection dynode is half way up in a 25 dynode chain with the circuitousdistance to the 25^(th) dynode being about 120 mm, as shown in column 3and as computed using an inter-dynode spacing of 10 mm. First, theacceleration field, as stated above, between any two dynodes for thisexample is chosen to be about 70 volts so as to result in an electronvelocity (i.e., for 70 eV electrons) at about 4.96E06 meters per second(m/s). Thus, knowing the circuitous distance to the last dynode as shownin column 3, and knowing the travel velocity for the signal electrons tobe collected at the anode, the computed response time, as shown incolumn 4 of Table 1, is about 24 nanoseconds (ns). Specific to thisexample, 24 ns is the critical time for the control circuit 12, of FIG.1A to provide the 25^(th) dynode with a regulating voltage if necessaryupon receiving a detection signal from the novel partitioned 13^(th)dynode.

Knowing the response time, the output load current to switch the voltageof the 25^(th) dynode is given by Equation 2:I=CdV/dt;  (2)with C being the coupled capacitance and dV/dt being a slew raterequired to switch the voltage at, for example, the 25^(th) dynode.Using 370 pF as the example capacitance and the slew rate dV/dt of 30volts in the computed response time of 24 ns, the resultant currentrequired by the 25^(th) dynode is about 0.46 Amperes (A), as shown incolumn 4 of Table 1 for the 13^(th) dynode.

Accordingly, if the 13^(th) dynode chosen in this example provides apredetermined saturation threshold current using a 10% value of theavailable current so as to trigger then the control circuit, the controlcircuit can then regulate the voltage at the 25^(th) dynode via a highvoltage power supply (not shown)/operational amplifiers 20, as shown inFIG. 1A, that can provide a slew rate of 1.24 kV/psec, as shown incolumn 6 of Table 1.

To compute the trigger 10% value of the available current at anyintermediate dynode DI, e.g., the 13^(th) dynode, one can use Equation3:I _(Dn)=(G)^(n-1) I _(DI);  (3)with G being the inter-dynode gain, I_(Dn) being the current at thedownstream Dynode n, I_(DI) being the available current at anintermediate dynode of the present invention, and n−1 being the numberof dynodes that precedes the intermediate dynode.

Using the 13^(th) dynode as the example intermediate dynode, Equation 3becomes Equation 4:I _(D25)=(G)¹² I _(D13);  (4)which can be rearranged to provide Equation 5:I _(D13) =I _(D25)/(G)¹².  (5)Note: because the total gain for the 25 dynode chain is given herein as10⁶, the individual dynode gain G=(10⁶)^(1/25)=1.737.

Thus, given a known deleterious example saturation current at the25^(th) dynode being about 1 mA, the corresponding threshold currentlevel at the 13^(th) dynode, which indicates saturation at the anode isgiven by solving Equation 5 above to result in:I _(D13)=1 mA/(1.737)¹²=1.3 μA.  (6)

Thus, using 10% of the above calculated saturation current for the Erroramplifier 18 of the control circuit 12, as shown in FIG. 1A, (e.g., viathe 10% partitioned portion of the 13^(th) dynode) results in about 130nA as the available trigger current capable of saturating the anode, asshown in column 2 (i.e., labeled as Available Current) of Table 1. It isthis level of current that if sensed for this example, operates toenable (i.e., trigger) the control circuitry 12 of FIG. 1A to regulatethe one or more downstream dynodes.

The operating parameters for the rest of Table 1 are similarly derivedwhen analyzing a particular intermediate dynode arrangement of thepresent invention if choosing a 10% partitioned available current value(i.e., by modifying the intermediate dynode with a ratio a ratio of90%-10%. FIG. 2 shows the results of Table 1 with respect to the slewrate 202 and available current 206 as plotted against the respectivedynode number using such constraints. Using an example plot, such as theplot shown in FIG. 2 enables a user of the present invention thecapability of optimizing a given configuration by knowing predeterminedsystem limits. As shown labeled at the top of FIG. 2, detecting currentslower than about 100 pA and a slew rate greater than about 5 kV are thesensitivity and slew rate figures of merit for existing discreethardware. Accordingly, for the example configuration discussed abovethat provides the operating parameters shown in FIG. 2, the electrodesfrom dynode 2 up to about dynode number 20 are also capable of beingutilized as the modified intermediate detection dynode.

It is to be understood that features described with regard to thevarious embodiments herein may be mixed and matched in any combinationwithout departing from the spirit and scope of the invention. Althoughdifferent selected embodiments have been illustrated and described indetail, it is to be appreciated that they are exemplary, and that avariety of substitutions and alterations are possible without departingfrom the spirit and scope of the present invention.

What is claimed is:
 1. A particle detector, comprising: a cathode thatemits electrons in response to receiving incident particles thatrepresent one or more input signals; a plurality of cascaded dynodesconfigured to provide a sensed current at an anode that is related tothe number of received incident particles; an interposed dynode arrangedas part of said plurality of cascaded dynodes, wherein said interposeddynode comprises partitioned sections to direct received electrons tothe remainder of said plurality of cascaded dynodes for furtheramplification and to further provide a detection current indicative ofthe magnitude of said one or more input signals; and a control circuitcoupled to one or more downstream dynodes within said plurality ofcascade dynodes and configured to receive said detection current so asto regulate the voltage gain to said one or more downstream dynodes uponsaid detection current meeting a predetermined current threshold.
 2. Theparticle detector of claim 1, wherein said interposed partitioned dynodecomprises unequal sections.
 3. The particle detector of claim 2, whereinsaid unequal sections comprises a first portion configured to directreceived electrons to the remainder of said plurality of cascadeddynodes for further amplification and a second portion to provide saiddetection current so as to regulate the voltage gain to said one or moredownstream dynodes.
 4. The particle detector of claim 2, wherein saidunequal sections comprises a ratio of 90%-10%, wherein said 90% portionis configured to direct received electrons to the remainder of saidplurality of cascaded dynodes for further amplification and said 10% isconfigured to provide said detection current so as to regulate thevoltage gain to said one or more downstream dynodes.
 5. The particledetector of claim 1, wherein said interposed dynode comprises aninterstitial electrode configured at a first potential to receive anddirect received electrons to the remainder of said plurality of cascadeddynodes for further amplification while an adjacently coupled electrodeat a different potential from said mesh electrode receives thoseelectrons that are directed through said interstitial electrode toprovide for said detection current.
 6. The particle detector of claim 1,wherein said interposed partitioned dynode comprises equal sections thatfurther includes: a first portion configured to direct receivedelectrons to the remainder of said plurality of cascaded dynodes forfurther amplification and a second portion to provide said detectioncurrent so as to regulate the voltage gain to said one or moredownstream dynodes.
 7. The particle detector of claim 1, wherein saidinterposed dynode comprises at least one electrode structure selectedfrom: a spherical structure, a cylindrical structure, a mesh structure,a planar structure, and a curved structure.
 8. The particle detector ofclaim 1, wherein said plurality of dynodes comprises from at least 13dynodes up to about 40 dynodes.
 9. The particle detector of claim 1,wherein said downstream dynodes to be regulated are adjacent to saidanode.
 10. The particle detector of claim 1, wherein said downstreamdynodes comprises a slew rate of less than about 5 kV/μs.
 11. Theparticle detector of claim 1, wherein an available detection currentprovided by said interposed partitioned dynode is at least 100 pA. 12.The particle detector of claim 1, wherein said incident particlescomprise at least one of charged particles, neutral particles, andphotons.
 13. The particle detector of claim 1, wherein said detectorcomprises a photomultiplier.
 14. The particle detector of claim 1,wherein said plurality of cascaded dynodes is coupled to a resistivenetwork.
 15. The particle detector of claim 1, wherein said controlcircuit comprises discreet circuitry configured to provide a controlsignal prior to the arrival of the propagating amplified electronsdirected from dynode to dynode.
 16. The particle detector of claim 1,wherein said particle detector can detect particles provided by at leastone mass spectrometer instrument selected from: a time of flightinstrument (TOF) and a quadrupole electrode device.